A hydrofluoroolefin such as 2,3,3,3-tetrafluoropropene (CF3CF═CH2) (hereinafter sometimes referred to as “HFO-1234yf) is useful as an alternative compound to a chlorofluorocarbon to be used as a refrigerant, etc., since the hydrofluoroolefin contains no chlorine atom.
As a method for producing the hydrofluoroolefin, for example, the following methods (i) to (iv) have been proposed.
(i) A method of subjecting 1,1-dichloro-2,2,3,3,3-pentafluoropropane (CF3CF2CHCl2) (hereinafter sometimes referred to as “HCFC-225ca”) to a dehydrofluorination reaction to obtain 1,1-dichloro-2,3,3,3-tetrafluoropropene (CF3CF═CCl2) (hereinafter sometimes referred to as “CFO-1214ya”), and then reacting CFO-1214ya with hydrogen as represented by the following formula (6) in the presence of a palladium catalyst supported on alumina to obtain HFO-1234yf (Patent Document 1).CF3CF═CCl2+2H2→CF3CF═CH2+2HCl  (6)
(ii) A method of subjecting RfCF═CX2 (wherein Rf is a C1-10 fluoroalkyl group, and X is a chlorine atom, a bromine atom or an iodine atom) and hydrogen to a reaction as represented by the following formula (9) in the presence of a palladium catalyst supported on activated carbon to obtain RfCF═CH2 (Patent Document 2).RfCF═CX2+2H2→RfCF═CH2+2HX  (9)
(iii) A method of subjecting chlorotrifluoroethylene and hydrogen to a reaction as represented by the following formula (10) in the presence of a palladium catalyst supported on activated carbon to obtain a trifluoroethylene. (Patent Document 3).CF2═CClF+H2→CF2═CHF+HCl  (10)
(iv) A method of subjecting 1,2-dichlorodifluoroethylene and hydrogen to a reaction as represented by the following formula (11) in the presence of a palladium catalyst supported on activated carbon (Patent Document 4).CClF═CClF+2H2→CHF═CHF+2HCl  (11)
However, in the method (i), 1,1,1,1,2-tetrafluoropropane (CF3CHFCH3) (hereinafter sometimes referred to as “HFC-254eb”) which is an over-reduced product having hydrogens added to HFO-1234yf and 3,3,3-trifluoropropene (CF3CH═CH2) (hereinafter sometimes referred to as “HFO-1243zf”) which is an over-reduced product having some of fluorine atoms in HFO-1234yf replaced by hydrogen atoms, are formed as by-products.
If the over-reduced product is formed in a large amount, the yield of the aimed product decreases, and the production efficiency decreases. Further, HFO-1243zf has a boiling point close to that of the aimed HFO-1234yf, and is thereby hardly separated and removed by subsequent distillation. Accordingly, if a large amount of HFO-1234zf is by-produced, HFO-1243zf will remain as an impurity in HFO-1234yf obtainable by distillation. In such a case, a separation and purification step is additionally required to obtain high purity HFO-1234yf.
Also in the method (ii), RfCHFCH3 and RfCH═CH2 which are over-reduced products are formed as by-products together with the aimed RfCF═CH2. In a case where Rf is CF3—, that is, in a case where the aimed product is HFO-1234yf, the production efficiency of HFO-1234yf decreases, and a separation and purification step is additionally required to obtain high purity HFO-1234yf, in the same manner as in the method (i).
Also in the methods (iii) and (iv), in addition to the aimed product, an over-reduced product is formed as a by-product, whereby the production efficiency of the aimed product decreases, and the purity of the aimed product decreases.